16. Hydroxy compounds

Alcohol preparation routes include steam addition to an alkene, cold dilute acidified manganate(VII) addition to form a diol, aqueous hydroxide substitution of a halogenoalkane, and NaBH₄/LiAlH₄ reduction of an aldehyde or ketone.

Choose the route from the starting functional group and state the conditions. The product class and carbon skeleton must match the reaction pathway.

Ethene + steam ⇌ ethanol with H₃PO₄; bromoethane + OH⁻(aq) → ethanol; ethanal + NaBH₄ → ethanol. Three routes reach the same alcohol from different precursors.

Cold manganate(VII) gives a diol, not the same product as hot oxidative cleavage, and aqueous OH⁻ is not ethanolic elimination reagent.

Alcohols burn in oxygen to form CO₂ and H₂O when combustion is complete. Their –OH group can be replaced to form a halogenoalkane, and sodium removes the acidic O–H hydrogen to form an alkoxide and H₂.

Use the reagent to identify the reaction: HX or a phosphorus/ thionyl chloride reagent gives substitution, while sodium gives an acid–metal reaction. The carbon skeleton usually remains unchanged.

2ROH + 2Na → 2RONa + H₂. Ethanol + PCl₅ → chloroethane + POCl₃ + HCl, illustrating conversion of –OH into a leaving group.

Alcohols are weak acids, not hydroxide salts, and sodium reacts at the O–H bond rather than replacing the whole carbon chain.

A primary alcohol has –OH on a carbon attached to one carbon, secondary to two, and tertiary to three. Acidified dichromate(VI) oxidises primary and secondary alcohols but tertiary alcohols resist mild oxidation.

On warming, orange Cr₂O₇²⁻ is reduced to green Cr³⁺ when oxidation occurs. Classification predicts the likely oxidation product, but the colour test alone does not name the alcohol.

Propan-1-ol is primary and can oxidise to propanal then propanoic acid; propan-2-ol is secondary and oxidises to propanone; tert-butanol is tertiary and shows no reaction under these conditions.

Primary/secondary/tertiary refers to the carbon attached to OH, not the number of OH groups or total carbons.

An alcohol containing CH₃CH(OH)– is oxidised under alkaline iodine conditions to a methyl-carbonyl intermediate, which gives a yellow precipitate of triiodomethane, CHI₃.

The test also applies to compounds that can form the same CH₃CO– pattern after oxidation. Identify the structural motif before using the observation as evidence.

Ethanol gives the yellow iodoform precipitate because it oxidises to ethanal, which contains the required methyl-carbonyl group. Propan-1-ol does not give this test.

A yellow precipitate is not a general alcohol test; it points to the specific methyl-containing motif.

An alcohol can donate O–H hydrogen, but its conjugate base RO⁻ is less stabilised than hydroxide in water. Alcohols therefore ionise only slightly and are weaker acids than water.

The C–O bond and alkyl groups influence electron density, but the key comparison is the equilibrium and conjugate-base stability. Sodium reacts because it removes the acidic O–H hydrogen, not because alcohol is a strong acid.

2ROH + 2Na → 2RONa + H₂, while an alcohol does not neutralise aqueous sodium hydroxide as a typical acid would.

“Weak acid” does not mean no reaction; it means a small equilibrium ionisation under the stated solvent conditions.